`a)` Vì `O` là trung điểm của `AC;BD`

    `=>{(\vec{OA}=-\vec{OC}),(\vec{OB}=-\vec{OD}):}`

Ta có: `\vec{OA}+\vec{OB}+\vec{OC}+\vec{OD}`

    `=-\vec{OC}-\vec{OD}+\vec{OC}+\vec{OD}=0`

`b)` Vì `E` là trung điểm `AD=>\vec{EA}=-\vec{ED}`

Ta có: `\vec{EA}+\vec{EB}+2\vec{EC}`

`=\vec{EA}+\vec{EA}+\vec{AB}+2\vec{ED}+2\vec{DC}`

`=-2\vec{ED}+\vec{AB}+2\vec{ED}+2\vec{AB}=3\vec{AB}`

`c)` Ta có: `\vec{EB}+2\vec{EA}+4\vec{ED}`

`=\vec{EB}-2\vec{ED}+4\vec{ED}`

`=\vec{EB}+2\vec{ED}`

`=\vec{EA}+\vec{AB}+2\vec{ED}`

`=-\vec{ED}+\vec{AB}+2\vec{ED}`

`=\vec{AB}+\vec{EC}+\vec{CD}` (Mà `\vec{AB}=-\vec{CD}`)

`=\vec{EC}`

Câu 2

Đổi : 72 km/h =20 m/s
Gia tốc của ô tô là

\(a=\dfrac{v^2-v_0^2}{2s}=\dfrac{20^2-4^2}{2\cdot400}=0,48\left(\dfrac{m}{s^2}\right)\)

Câu 3:

\(\dfrac{F_1}{F_2}=\dfrac{d_2}{d_1}\Rightarrow d_2=\dfrac{F_1\cdot d_1}{F_2}=\dfrac{30\cdot6}{40}=4,5\left(cm\right)\)

Khoảng cách giữa lực F1 và F2

\(d=d_1+d_2=6+4,5=10,5\left(cm\right)\)

C-C

a: Thay x=2 và y=3 vào hàm số, ta được:

\(2k+2-4=3\)

\(\Leftrightarrow k=\dfrac{5}{2}\)

1e 2h 3f 4a 5g 6c 7b 8d

1 e

2 h

3 f

4 a

5 g

6 c

7 b

8 d

cái này thì triệt tiêu ra đó bn

là bằng 2020/6069

A = 1/3.5.7 + 1/5.7.9+ ... + 1/2019.2021.2023

2A = 2/3.5.7 + 2/5.7.9+ ... + 2/2019.2021.2023

2A = 1/3-1/5+1/7+1/5-1/7+1/9+....+1/2019-1/2021+1/2023

2A = 1/3 - 1/2023

2A = 2023/6069 - 3/6069

2A = 2023-3/6069

2A = 2020/6069

 A = 1010/6069

Vậy A = 1010/6069

62 The more I think about the idea, the less attractive it becomes

63 The more slowly you walk, the longer it takes to get there

64 The less right you treat your disease, the more increasingly serious it will become

65 câu này thiếu dữ liệu roài

66 The newer the motorbike is, the less petrol it consumes

67 The more electricity you use, the higher your bill will be

68 The more books he reads, the more he knows about the word

69 The more I thought about , the more confused I became

70 The more problems arise, the less enthusiastic we feel

71 The more polluted the air is, the greater the chances of developing respiratory diseases are

72 The more popular TV programs become, the worse they seem to get

73 The less my boss is pressed for time, the worse he works

74 The more the economy grows, the better people's living conditions become

75 If you go to bed late, you will be tired

76 Unless you spend all your money, we can go on holiday

77 If you don't study, you won't pass your exams

78 Unless he smokes, the house won't smell bad

79 If you don't put your clothes away, your room will bea mess

80 If you don't go shopping, there won't be any food in the fridge

81 Unless you drink too much coffee, you can sleep

82 If you don't stop talking, you won't understand the lesson

83 If she studies hard, she will pass the exams

84 Unless we hurry up, we will be late for the meeting

85 If I had free time, I could help my mother with the housework

86 If he had his homework, he wouldn't be punished by the teacher

87 Unless he had a terrible headache, she wouldn't be absent from her class

88 If I knew her number, I would ring her up

89 Unless you rest yourself, you will be ill

90 Unless he ate so many chips, he wouldn't be fat

     Now, many people are aiming for a good life so they have moved to the city, which is fully furnished. So what are the prerequisites for a city to be liveable living? A city needs the following conditions. A city needs full convergence of factors that make it attract domestic and foreign tourists not only want to come here to visit but also want to choose this place as a residence. It fully converges the charm of visitors to this place. That city should have so many things to choose from. Because beauty is not only the scenery, beauty also comes from the heart of humanitarian cancer hospitals, with wifi network covering the whole city, with delicious, cheap, impressive food and also with long white sand beaches. poetic not to waste. Next is the climate, the city deserves to live so it has a cool climate, not too hot or too cold. And one more condition for a city to be sustainable, it must be free from theft, social evils. Above are my opinions on the conditions for a city to be sustainable and liveable living, I hope everyone can find a city worth living like that to live happily their whole life.